The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to self-destructing fluid-loss control pills for fluid-loss control in drilling, completion, and stimulation applications and their associated methods. “Fluid loss” refers to the undesirable migration or loss of fluids (such as the fluid portion of a drilling mud or cement slurry) into a subterranean formation.
Providing effective fluid-loss control for well treatment fluids is highly desirable. A “well treatment fluid” is a fluid used in a subterranean application. The term “treatment” herein does not imply any particular action by the fluid or any component thereof. Fluid-loss control materials are additives specifically designed to lower the volume of a filtrate that passes through a filter medium. Most attain their fluid-loss control from the presence of solvent-specific solids, or from hydrated linear polymers that rely on filter cake backup and on viscoelasticity to inhibit flow into and through the formation. A variety of fluid-loss control materials have been used and evaluated, including foams, oil-soluble resins, acid-soluble particulates, graded salt slurries, linear viscoelastic polymers, and heavy metal-crosslinked polymers. Their respective comparative effects are well documented. Other techniques that have been developed to control fluid loss include the use of “fluid-loss control pills,” which sometimes are referred to as “lost circulation pills.” A “fluid-loss control pill” is a gelled fluid that is designed or used to provide some degree of fluid-loss control. Through a combination of viscosity, solids bridging, and cake buildup on the porous rock, these pills oftentimes are able to substantially seal off portions of the formation from fluid loss. They also generally enhance filter-cake buildup on the face of the formation to inhibit fluid flow into the formation from the well bore.
Typically, fluid-loss control pills comprise an aqueous base fluid and a high concentration of a gelling agent polymer (usually crosslinked), and sometimes, bridging particles, like graded sand, potassium salts, or sized calcium carbonate particles. The term “gel,” as used herein, and its derivatives refers to a semi-solid, jelly-like state assumed by some colloidal dispersions. The most commonly used fluid-loss control pills contain high concentrations (100 to 150 lbs/1000 gal) of hydroxyethylcellulose (“HEC”). HEC is generally accepted as a gelling agent affording minimal permeability damage during completion operations. Normally, HEC polymer solutions do not form rigid gels, but control fluid loss by a viscosity-regulated or filtration mechanism. Some other gelling agent polymers that have been used include guar, guar derivatives, carboxymethylhydroxyethylcellulose (“CMHEC”), and even starch.
As an alternative to linear polymeric gels for fluid-loss control pills, crosslinked gels often are used. Crosslinking the gelling agent polymer creates a gel structure that can support solids as well as provide fluid-loss control. Further, crosslinked fluid-loss control pills have demonstrated that they require relatively limited invasion of the formation face to be fully effective. To crosslink the gelling agent polymers, a suitable crosslinking agent that comprises polyvalent metal ions is used. Aluminum, titanium, and zirconium are common examples.
A preferred crosslinkable gelling agent for fluid-loss control pills are graft copolymers of a hydroxyalkyl cellulose, guar, or hydroxypropyl guar that are prepared by a redox reaction with vinyl phosphonic acid. The gel is formed by hydrating the graft copolymer in an aqueous fluid containing at least a trace amount of at least one divalent cation. The gel is crosslinked by the addition of a Lewis base or Bronsted-Lowrey base so that pH of the fluid is adjusted from slightly acidic to slightly basic. Preferably, the chosen base is substantially free of polyvalent metal ions. The resulting crosslinked gel demonstrates shear-thinning and rehealing properties that provide relatively easy pumping, while the rehealed gel provides good fluid-loss control upon placement. This gel can be broken by reducing the pH of the fluid. Some fluid-loss pills of this type are described in U.S. Pat. No. 5,304,620, assigned to Halliburton Energy Sources, the relevant disclosure of which is incorporated herein by reference. Fluid-loss control pills of this type are commercially available under the trade name “K-MAX” from Halliburton Energy Services in Duncan, Okla.
After their application, fluid-loss control pills can cause severe damage to near-well bore areas due to polymer filtration or filter-cake formation. At some point in the completion operation, the filter cake resulting from a fluid-loss control pill must be removed to restore the formation's permeability, preferably to at least its original level. If the formation permeability is not restored to its original level, production levels can be significantly reduced.
Polymer-based fluid-loss control pills often require long cleanup periods. Moreover, an effective cleanup usually requires fluid circulation to provide high driving force, which allows diffusion to take place to help dissolve the concentrated build up of materials. Such fluid circulation may not be feasible. Additionally, in lower temperature wells (i.e., those below about 80° F.), it is often difficult to find an internal breaker for the fluid-loss control pills that will work effectively. Although conventional breakers (both internal and external) tend to work very quickly, usually within hours, they often do not provide predictable breaks, and thus there is little control over the break. As a result, secondary precipitation materials may result that may be damaging to the formation. The term “break” (and its derivatives) as used herein refers to a reduction in the viscosity of the fluid-loss control pill, e.g., by the breaking or reversing of the crosslinks between polymer molecules, or some breaking of the gelling agent polymers. No particular mechanism is implied by the term. Another conventional method of cleaning up a fluid-loss control pill is to add a spot of a strong acid (e.g., 10% to 15% hydrochloric acid) with coiled tubing, which is expensive and can result in hazardous conditions.